1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotography-basis copying machine or laser printer. More particularly, the invention relates to an image forming apparatus for forming an image by use of belt-like image forming members, e.g., a belt-like photosensitive member and a belt-like intermediate transfer member.
2. Description of the Related Art
Such an image forming apparatus as an electrophotography-basis copying machine or printer employs an image forming process in which an unfixed toner image is formed on an electrostatic latent image carrier, e.g., a photosensitive drum, and transferred from the image carrier onto a recording medium, e.g., a piece of paper, whereby a picture is printed on the recording medium. To transfer the unfixed toner image to the recording medium, two methods have been used. A first method is to directly transfer the unfixed toner image onto the recording medium. A second method is to primarily transfer an unfixed toner image from the electrostatic latent image carrier onto an intermediate transfer member shaped like a drum or an endless belt film, and then to secondarily transfer the unfixed toner image from the intermediate transfer member to a recording medium, whereby an image on an original document is reproduced on the recording medium.
FIG. 13 is a diagram schematically showing a model of a color printer which is an example of the image forming apparatus using the belt-like intermediate transfer belt. As shown, the surface of an electrostatic latent image carrier (which takes the form of a photosensitive drum in the description of the specification) 1 is uniformly charged by a charger 2, and scanned with a laser beam L, so that an electrostatic latent image defined by an image signal of a first color is formed on the surface of the latent image carrier. As the photosensitive drum I is rotated in the direction A with an arrow head, the latent image moves and reaches a position facing a developing device of the first color of a developing unit 3, and it is developed into a toner image T by the developing device of the first color. The photosensitive drum 1 carrying the toner image T thereon is further rotated.
An intermediate transfer belt 4 moves at a speed substantially equal to the peripheral speed of the photosensitive drum 1 in harmony with the toner developing operation. In a primary image transfer section in which a primary image transfer roll 5 is disposed in contact with the intermediate transfer belt 4 right under a position where the photosensitive drum 1 comes in contact with the intermediate transfer belt 4, the toner image T is primarily transferred from the photosensitive drum 1 onto the intermediate transfer belt 4 under a transfer electric field which is applied to the primary image transfer roll 5, the polarity of the transfer electric field being opposite to the toner charging polarity. Here, a primary image transferring cycle is completed.
As the intermediate transfer belt 4 travels, the toner image primarily transferred onto the intermediate transfer belt 4 moves and reaches a secondary image transfer section including a secondary image transfer roll 6 disposed therein. In the full-color image forming apparatus, the process ranging from the latent image forming step to the primary image transfer step is repeated a predetermined number of times (generally, for four colors of yellow (Y), magenta (M), cyan (C) and black (Bk)), whereby toner images of a multiple of colors are superposed on the intermediate transfer belt 4 to form a full-color toner image thereon.
To form an image of a multiple of colors, the developing unit 3 is constructed with a rotary machine including four color developing devices; yellow, magenta, cyan and black developing devices 3-1 to 3-4. The developing unit 3 thus constructed sequentially develops the latent images of those colors that are formed on the photosensitive drum 1 into color toner images.
After the toner image of the first color that is carried on the photosensitive drum 1 is transferred from the drum onto the intermediate transfer belt 4 in the primary image transfer section, the following steps are executed: toner left on the surface of the photosensitive drum 1 is removed by a cleaner 7; the surface is neutralized by a discharger (not shown); and an electrostatic latent image for the second color is formed thereon. The latent image of the second color is developed into a toner image of the second color in a similar manner to that in which the latent image of the first color is developed. The toner image of the second color is transferred on the first toner image already transferred on the intermediate transfer belt 4, in a superposing fashion. The toner images of the third and subsequent colors are sequentially superposed on the toner images previously transferred onto the intermediate transfer belt 4. Finally, a multi-color toner image, not yet fixed, which is formed by superposing those toner images on another, is formed on the intermediate transfer belt 4.
A recording medium, e.g., a recording paper P, is fed from a paper tray 8 to the secondary image transfer section at an instant that the intermediate transfer belt 4 carrying the multi-color toner image primarily transferred thereto reaches the secondary image transfer section.
When the recording paper P is transported in a state that it is nipped between the secondary image transfer roll 6 and the intermediate transfer belt 4, the toner image is secondarily transferred from the intermediate transfer belt 4 to the recording paper P under a transfer electric field developed by a transfer voltage applied to the secondary image transfer roll 6, the polarity of the transfer voltage being opposite to the charging polarity of the toner image.
The recording paper P having the toner image secondarily transferred thereonto is transported to a fixing unit 9. The fixing unit 9 heats, under pressure, the toner image to fix it onto the recording paper P. Here, an image forming process is completed. A discharger (not shown) for discharging the recording paper P having the toner image secondarily transferred thereto is located downstream of the secondary image transfer roll 6.
The secondary image transfer roll 6 is movable to and from the intermediate transfer belt 4 in the directions C with arrow heads. The secondary image transfer roll 6 comes in contact with the intermediate transfer belt 4 when the recording paper P approaches to the secondary image transfer roll 6, and it detaches from the intermediate transfer belt 4 when the recording paper P leaves the secondary image transfer roll 6. Upon completion of the secondary image transferring operation, the secondary image transfer roll 6 returns to its stand-by position. A cleaner 10, which is disposed facing the intermediate transfer belt 4, moves to and from the intermediate transfer belt 4 as the secondary image transfer roll 6 so does, to remove the toner image that is left while being not transferred to the recording paper P.
The intermediate transfer belt 4 passes around a drive roll 11, an idle roll 12, a secondary-image-transfer back-up roll 13 and a tension roll 14, and is transported in the direction B with an arrow head by the drive roll 11. The intermediate transfer belt 4 is provided with a control member (not shown) to control the position of the intermediate transfer belt 4 on various rolls, e.g., the drive roll 11 when viewed in the axial direction of each roll.
The surface of the drive roll 11 is coated with a high friction material to prevent the intermediate transfer belt 4 from slipping on the drive roll 11 when loads by the cleaner 10 and the secondary image transfer roll 6 are imparted onto the intermediate transfer belt 4.
In the color image forming apparatus using the intermediate transfer belt, a composite toner image (formed by superposedly transferring color toner images) is transferred onto the recording medium. Therefore, it can effectively prevent the misregistration among the color toner images and the image turbulence, which are essential to the image transfer method in which the color toner images are directly successively transferred from the latent image carrier onto the recording medium.
There are various proposals to prevent a variation of the peripheral speed of the intermediate transfer belt 4 when it travels, to control the intermediate transfer belt 4 to a predetermined position on the drive roll 11, for example, in its axial direction and to prevent breakage occurring at the ends of the intermediate transfer belt 4.
One of the proposals is disclosed in Japanese Patent Unexamined Publication No. Hei. 2-27383. In the proposal, a rib is provided on one of the ends of the intermediate transfer belt which is located outside the image forming area on the intermediate transfer belt. Grooves are formed in the related rolls including the drive roll, in association with that rib. A coefficient of friction of the rib is selected to be different from that of the intermediate transfer belt. Another proposal is disclosed in Japanese Patent Unexamined Publication No. Hei. 4-257888. In this proposal, ribs are provided on both ends of the belt passing about the drive roll and the follower roll. Grooves are formed in both ends of the drive roll and the follower roll, in association with the ribs.
Japanese Patent Unexamined Publication No. Hei. 5-134556 discloses yet another proposal. The publication discloses such a transfer belt in which a tape as a reinforcing member is bonded to the end of the transfer belt. The outside diameter of the roll is reduced at the portion thereof corresponding in position to the tape to prevent crack occurring at the boundary between the transfer belt and the reinforcing member by suppressing the rising of the transfer belt at a location where the roll is in contact with the bonded tape.
An additional proposal is disclosed in Japanese Patent Unexamined Publication Nos. Hei. 9-175686 and Hei. 9-16512. In this proposal, the transfer belt and the rib are joined together by stitching to prevent the rib from slipping off the transfer belt.
A technique for preventing the intermediate transfer belt from slipping on the drive roll is disclosed in Japanese Patent Unexamined Publication No. Hei. 6-35331. In this publication, the surface of the drive roll is made irregular in height ranging from 20 to 100 .mu.m. Another slip-preventing technique is disclosed in Japanese Patent Unexamined Publication No. Hei. 8-152812. In this publication, the inner surface of the intermediate transfer belt and/or the surface of the drive roll is entirely or partly coated with adhesive or high friction resin.
In the image forming apparatus which includes the intermediate transfer belt and the drive roll for driving it, the rib for preventing a zig-zag motion of the intermediate transfer belt, and the rib guide, the intermediate transfer belt is a semiconductive film of 50 to 100 .mu.m thick, which consists of a base of polycarbonate or polyimide resin and a resistance adjusting material. The surface of the drive roll is generally processed to have a high friction to prevent slippage between the surfaces of it and the belt.
For the process to secure the high friction surface of the drive roll, the surface of an aluminum roll is coated with high friction resin of, for example, urethane rubber, whereby the drive roll and the intermediate transfer belt maintain their high friction for a long time. However, when the drive roll and the belt are new or nearly new, the friction coefficient of the surface of the drive roll is excessively high, and the belt repeats the stick slip on the drive roll in its axial direction to possibly squeak.
During when the intermediate transfer belt passing around a plurality of rolls travels, it moves in the axial direction of the roll (this motion is called a walk). The combination of the rib and the rib guide controls the walk so as not to be in excess of a predetermined amount of walk. However, when a state that the rib and the rib guide push each other by the walk continues for a long time, the ends of the intermediate transfer belt, if not sufficient in mechanical strength, will be broken, and if broken, the image forming apparatus fails to continue its image forming operation.
Such a strong force as to break the intermediate transfer belt is caused by degradation of the flatness of the belt system, which is due to poor levelness of the apparatus per se, twists caused by the stacking of component parts on the front and rear side plates of the apparatus per se and assembling errors, the circumferential length difference between both sides of the ends of the belt, and the like. However, there is a possibility that the intermediate transfer belt as the image carrier in the image forming apparatus is broken by the cause proper to the intermediate transfer belt.
A mechanism to break the intermediate transfer belt will be described. The combination of a new drive roll and a new intermediate transfer belt has a high coefficient of friction, and hence a high gripping force is also created. Therefore, when the rolls supporting the intermediate transfer belt lose their alignment (parallelism of the axes of the rolls), the belt is liable to walk even if the misalignment is slight. In this case, the moving belt shifts sideways for a short time or after it has traveled several tens cycles, and the rib abuts against the rib guide by a strong gripping force.
At a position where the belt is put on the drive roll and at a position where the belt leaves the drive roll, the following forces act on the side face of the rib. FIG. 14 is a cross sectional view showing the intermediate transfer belt 4 put on the drive roll 11, and FIG. 15 is a cross sectional view showing a contact state of the drive roll 11 with the intermediate transfer belt 4. In those figures, to prevent the walk of the intermediate transfer belt 4, ribs 41 are provided on both sides of the back surface of the intermediate transfer belt 4 in a state that it is in contact with the side faces of the drive roll 11.
When the intermediate transfer belt 4 walks and comes in contact with the side face of the drive roll 11, a force F1 acts on the side face of the rib 41 in a region R1 in which the intermediate transfer belt 4 begins to contact with the drive roll 11. The force F1 acts so as to cause the intermediate transfer belt 4 to rise and run onto the drive roll 11. In a region R2 where the lifted intermediate transfer belt 4 leaves the drive roll 11, a force F2 acts on the side face of the rib 41, so that the intermediate transfer belt 4 descends to its normal level.
FIG. 17 is a cross sectional view showing the intermediate transfer belt 4 when it suffers from a rise. The intermediate transfer belt 4 is lifted by the force F1, while at the same time a strong pushing force acts on the side face of the rib 41, whereby a rise RU is formed. This rise RU disappears in the region R2. In the vicinity of the drive roll 11, the side ends of the intermediate transfer belt 4 are repetitively deformed alternately in one direction and the other direction that is opposite to the former: the side ends of the belt are repetitively subjected to an alternate process of the concentration and release of stress.
The force that presses the rib 41 against the side face of the drive roll 11 increases as the gripping force is larger and the alignment decreases in its accuracy, and when this force is large, the intermediate transfer belt 4 is liable to rise. When the rib 41 is pressed against the side face of the drive roll 11, the intermediate transfer belt 4 is subjected to the alternate process of the stress concentration and release, and further the intermediate transfer belt 4 rubs with the drive roll 11 to generate squeak.
If the alternate concentration and release of stress is repeated in the rise portion RU and in this state the operation of the image forming apparatus continues, a fatigue is accumulated in the rise portion RU to give rise to a crack CR (FIG. 18). The local crack CR grows into a breakage of the whole intermediate transfer belt 4. Further, there is a danger that a notch N of the end of the intermediate transfer belt 4 easily grows to the breakage of the whole intermediate transfer belt 4 (FIG. 19).
To prevent the walk of the intermediate transfer belt, it is necessary to secure accurate working of component parts and assembling of them. To this end, it is required that the rolls supporting the intermediate transfer belt are exactly aligned to one another, the intermediate transfer belt is accurately worked to have little difference of its circumferential length between the sides of the ends of the belt.
The approach of improving the mechanical precision of the intermediate transfer belt and its related rolls brings about the complexity of the steps of working, assembling and adjustment. In this respect, the approach is not suitable for the mass production of the image forming apparatuses. If the problems in the manufacturing stage are solved, the following problem is still present; when the image forming apparatus is installed on a place of poor levelness, it is impossible to secure the required accuracy of the alignment among the rolls that support the intermediate transfer belt.
To prevent the walk problem, it is necessary to strictly manage the precision of the component parts and assembling of them as stated above. Further, some measure for improvement must be taken for other factors that will cause the walk producing the strong pushing force, e.g., gripping force.
Further, as described above, the conventional technique reinforces the belt ends by use of a tape or the like applied thereto, to thereby protect the belt from being cracked. This belt protecting technique has the following disadvantage, however. The reinforcing tape is deformed every time the intermediate transfer belt passes each roll supporting the former, and will be peeled from the belt ends.
Furthermore, as described above, the toner images of different colors are composed by superposing these color toner images one on another. Therefore, it is essential to accurately register those color toner images or to prevent a misregistration of those color toner images (referred to frequently as a color misregistration). To this end, it is necessary to accurately detect the reference position on the intermediate transfer belt and to control the operations of the related portions in the image forming apparatus in accordance with the detecting signal indicative of the reference position.
To detect the reference position, the conventional technique detects a paint or a tape on the intermediate transfer belt, reads a marking (e.g., a through-hole) on the belt by use of a reflection type sensor, or reads a rotation position on the drive roll for the belt by use of an encoder.
In the reference-position detecting method using the marking of the through-hole, stress concentrates at the through-hole, possibly cracking the intermediate transfer belt. In the detecting method of reading the rotation position of the drive roll, an error that arises from slippage between the belt and the drive roll is liable to occur. In this respect, the detecting method using the paint or tape is superior to the remaining detecting methods.
However, the method using the paint or tape has the following problem. To reduce the misregistration of the color toner images, it is necessary to detect the marking of the paint or tape considerably accurately. For example, when the color misregistration is reduced to 125 .mu.m, the marking detection error should be within 15 .mu.m.
To satisfy such a strict requirement, it is necessary to eliminate various factors causing detection errors, such as the moving speed of the intermediate transfer belt, the bending or vibration of the intermediate transfer belt during its traveling, and the mounting position of the reflection type sensor.